When a furnace is operated for melting mineral materials having high melting points, such as glass, the furnace must be capable of withstanding the effects of different environments. In particular, the furnace must be able to withstand the environment (i.e., exposure to air or nitrogen) during the start up operation of the furnace wherein the temperature will gradually increase from room temperature to a high operating temperature, perhaps as high as 3000.degree. F. At the high temperatures present in a glass melting furnace the glass attacks most materials, causing spalling and other forms of deterioration. Further, the start up operation often takes up to one week or more the furnace must be able to withstand this harsh environment. Also, the furnace must function efficiently at the high operating temperatures during the continuous melting operation.
In order to withstand the high operating temperatures, various parts of the furnace are normally made from refractory metals such as molybdenum, tungsten, titanium or zirconium. These refractory metals are mechanically and physically resistant to dimensional changes under the extreme temperature conditions of an operating glass melting furnace. It is especially desirable to have the electrodes of a glass melting furnace be made from these refractory metals. It is also desirable to have the furnace contain a wall made of a refractory metal in order to prevent contamination of the glass by deterioration of the refractory insulating brick wall.
During the start up operation of a glass melting furnace, however, the typical refractory metal experiences an environment which causes considerable deterioration. Normally, a glass furnace is started up by heating to a molten state a small amount of glass batch in the vicinity of a few of the electrodes. The power is then slowly increased to the electrodes and gradually the temperature, and the amount of the molten glass in the furnace, is increased. During this time, while the electrodes are being heated to nearly operational temperatures of about 3000.degree. F. the electrodes are partially exposed to the environment in the furnace, rather than being covered up with molten glass. The electrodes or other furnace parts are therefore subjected to extremely rapid deterioration.
Several methods have been developed for avoiding deterioration of the refractory metal parts due to the start up environment. One of these methods is to provide either an inert gas, such as nitrogen or argon, or a reducing gas, such as methane or hydrogen, to provide a protective envelope around the electrode. Another method involves heating the glass with temporary electrodes and/or products of combustion while keeping the operating refractory metal electrode cool during the furnace heating period. For example, a water-cooled jacket is placed around the electrode during the time in which the molten glass is built up around the electrode. When the protective water-cooled jacket is removed, the electrode experiences only molten glass and is not exposed to the atmosphere. However, these methods require extra equipment in the way of additional start up electrodes, or require somewhat complicated procedures, such as the removal of the protective water-cooled jacket.
Another method for avoiding deterioration of metal includes the use of an electrode or other refractory substrate made by alloying an iron/steel substrate with chromium. Still another well-known practice involves the use of nickelchromium alloys strengthened with oxides. However, the Applicant is unaware of any use of alloying chromium with molybdenum in order to avoid deterioration in the corrosive glass melting furnace environment.
Application of a protective coating to the electrode itself to ward off deterioration during start up has also been used to protect the electrodes. For example, a protective coating of molybdenum disilicide has been employed. However, the molybdenum disilicide coating has been found to provide only a few days of protection, whereas at least about one week of protection is required for glass melting furnace start up. In addition, corrosion protection of molybdenum using a Cr.sub.2 O.sub.3 coating applied by plasma torch has been attempted. For example, the Kithany U.S. Pat. No. 4,668,262 discloses corrosion protection of molybdenum from oxidation by application of a dual coating of molybdenum disilicide and chromium oxide with molybdenum disilicide as the intermediate coat. One of the advantages of the intermediate coating is better adhesion of chromium oxide to the molybdenum substrate. However, the dual coatings especially molybdenum disilicide, are difficult and expensive to apply. In addition, due to various heat transfer considerations, it is difficult to apply more than about a 25 mil thickness of the chromium oxide coating.
Thus, there is a need for an improved way to protect refractory metal parts in a furnace for melting materials at high temperatures during the start up operation.
There is also a need for refractory metal substrates having a protective coating of at least about 25 mil thickness for use in a furnace for melting materials at high temperatures during the start up operation of the furnace.